It is often desirable to measure the oxygen content of blood without taking a blood sample (e.g., for fetal monitoring during delivery). This can be done by measuring the oxygen released through the skin from capillaries lying adjacent the skin surface, the so-called transcutaneous partial pressure of oxygen (t.sub.c PO.sub.2). Conventionally the measurement is made by adhering to the skin an electrode having a membrane permeable to oxygen. One such electrode is disclosed in Eberhard et al., "An Introduction to Cutaneous Oxygen Monitoring in the Neonate," Hoffmann-La Roche & Co., AG (1976). Oxygen passes through the membrane and into an electrolyte region. The amount of oxygen is detected by measuring the current flowing through the electrolyte between an anode and a cathode. A heater in the electrode warms the skin to stimulate release of oxygen. The anode, cathode, electrolyte region, and heater are incorporated into a reusable unit. The membrane is removed after each use to add fresh electrolyte. As it is important to maintain the elevated temperature of the electrode within a narrow temperature range to assure measurement accuracy, the electrode is temperature calibrated.